The processing of various types of ore and other materials can be conducted in an acidic environment to produce a desired product. In wet process phosphoric acid production, a crude ore that has been initially processed by washing, desliming, flotation, grinding, or combinations thereof is digested in an acidic solution. After digestion is completed the resulting slurry is subjected to filtration followed by evaporation of the acid stream. Clarification is then performed to produce the desired product.
Wet process phosphoric acid production is typically performed under harsh conditions, such as high temperature, high phosphoric acid concentrations, and high soluble polyvalent ions such as Fe, Al, and others. As a result of these harsh conditions during processing of ores and other materials, scale deposits may form. The scale formation occurs primarily in the digesters, evaporators, and equipment associated therewith. A certain amount also occurs on other surfaces of the process including the filtration systems. Particularly troublesome is scaling of heat exchanger surfaces. Some attempts to correct this problem have been directed to equipment design but even the best-designed equipment is not capable of preventing scale formation. Further, the acidic and high multivalent cations environment typically render conventional scale inhibitors ineffective.
The primary by-product from the digestion of phosphate ore is calcium sulfate (CaSO4) and its hydrated variations. These primary by-products are typically the main source of scale deposition. There are different crystallographic forms of calcium sulfate responsible for scale deposition. These forms, calcium sulfate dihydrate or gypsum (CaSO4.2H2O), calcium sulfate hemihydrate (CaSO4.½H2O), and calcium sulfate anhydrite (CaSO4), are dependent on temperature and the residence time within the process. This phase transformation between forms adds to the complexity of their inhibition. Additional components of these scale depositions can include fluorosilicate salts, barium sulfate, calcium fluoride, or other materials depending on the composition of the process stream and specific process conditions.
Generally, the wet process production of phosphoric acid involves the digestion of a phosphate containing ore slurry with sulfuric acid. The resulting phosphoric acid is separated from precipitated calcium sulfate and other solid impurities by filtration. The phosphoric acid solution is then concentrated through multiple steps of evaporation and clarification to yield the finished phosphoric acid (50-80% phosphoric acid). Although much of the calcium sulfate and other impurities are removed during the filtration step, a significant amount remains dissolved in the process stream after filtration. As the phosphoric acid is concentrated through the evaporator circuit(s), calcium sulfate of various forms continues to precipitate from solution resulting in scale deposition on high temperature surfaces due to the low solubility of the scale and temperature at which the process is performed. This and other deposits negatively impact heat transfer to the process stream as well as restrict liquor flow. Consequently, the scale must be removed through periodic cleanouts. Thus, the scale deposition causes significant loss of process efficiency and results in added cost.
For example, scale deposition in the harsh operation conditions of phosphoric acid production (e.g., at a temperature of 60° C. and above, phosphoric acid concentration [H3PO4] at 42% and above, and Fe & Al ions as high as 8,000 ppm) can cause significant downtime and production loss associated with frequent process lines cleaning and descaling including belt filters, heat exchangers, and evaporators.
Accordingly, it is desirable to provide systems and methods for reducing formation of scale in phosphoric acid production. Furthermore, other desirable features and characteristics will become apparent from the subsequent summary and detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.